Adding support for instances (typed constants)

[Intuity]

This PR adds support for attaching instances of defined types to packages as typed constants:

• Extends the custom grammar to allow instances of types to be declared;
• Adds _pt_attach_instance method to Package, which can be used with the Python syntax to achieve the same thing;
• Extends unit tests to cover the new features;
• Extends package template to render instances.

[Copilot]

Pull Request Overview

This PR adds comprehensive support for instances (typed constants) to the packtype system, allowing users to declare and use structured constant values within packages.

• Adds new DeclInstance declaration type and _pt_attach_instance method to handle typed constants
• Extends grammar to support both simple enum instances and complex struct instances with field assignments
• Updates SystemVerilog template generation to properly render instance constants

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/grammar/test_instances.py	New comprehensive test suite covering enum and struct instance parsing
packtype/grammar/declarations.py	Adds DeclInstance, FieldAssignment, and FieldAssignments classes
packtype/grammar/transformer.py	Updates decl_alias method to handle instance declarations alongside aliases
packtype/grammar/packtype.lark	Extends grammar to support field assignments and instance syntax
packtype/grammar/grammar.py	Integrates instance processing into the main parsing pipeline
packtype/types/package.py	Adds _pt_attach_instance method and _pt_instances property
packtype/templates/package.sv.mako	Updates SystemVerilog template to render instance constants
packtype/utils/basic.py	Adds get_package utility and updates get_name for scalar types
packtype/common/expression.py	Adds support for foreign reference evaluation in expressions

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